The applicant of the present invention has acquired an United States Letter Pat. No. 4,709,565, titled "United Opening device for a double-locked door", which has been found to have the following shortcoming.
The main disc 4 connected with the main lock is provided with two projecting rings 42 on its two sides. Said rings 42 always keep in touch with the two hooks in the linking plate 1 so that said rings 42 are to be pressed once and to produce striking noise when the main disc 4 rotates. Besides, the spring 6 gradually loses elasticity pulling the main disc after a long period of use, which results in high malfunction.